1. Field of the Invention
This invention lies in the field of seismic sources for use in water covered areas. More particularly, my invention concerns a type of seismic source which while producing downwardly going acoustic waves, effectively prevents such waves from going upward and sideways from the source, thus eliminating or highly attenuating troublesome acoustic waves that reflect off the surface of the water or that travel horizontally within the water layer.
In the seismic exploration industry, important data required is the velocity or propagation of seismic waves in the underlying layers of the earth. In the practice of seismic prospecting this has generally been done by finding or creating a deep borehole that goes down to the depth for which velocity information is desired. A sonde is lowered by means of a conductor cable, which contains one or more seismic sensors. If three sensors are used, the axis of sensitivity of two sensors (or seismometers or geophones) are generally pointed in the orthogonal horizontal directions and the third one in the vertical direction.
A seismic wave source is provided at a selected depth in the water in the vicinity of the borehole. The seismic wave that is generated, on its way down into the earth, will pass by the sensor or seismometer suspended in the borehole at a selected depth. Thus, the overall travel time from the source to the well seismometer can be determined. Furthermore, energy that is directed downward and reaches an underlying geological interface, will be reflected upwardly and will also reach the well geophone. With this kind of information, and with the well geophone at selected different depths from below the surface to deep in the earth, a series of seismic traces can be provided which represent seismic energy received by the well geophone. These series of traces then provide a great amount of information regarding the type of elastic waves that travel up and down in the earth, such as compressional or shear waves for example, and of their velocities of propagation in the subsurface formation. This operation is known generally as vertical seismic profiling.
2. Comments Relating to Prior Art
In the prior art, there are patents and publications describing experiments and devices in which a screen of air or gas bubbles within a water body are provided to attenuate high level acoustic waves from passing through the water body, and through the volume which has the partial gas saturation. The art which I have uncovered is related to special cases where undesired acoustic waves or noise are attenuated. All the prior art discovered included solely the absorption and attenuation effect of the gas saturation volume of water. No art was found which related to the reflected and focusing of acoustic waves at the interface between the gas-water mixture volume and the remaining body of water.